In the same way as the human eye, light sensors, such as video cameras and CCDs, are only able to effectively process light up to a certain intensity. Very intense light sources, such as the sun, lightning bolts, or powerful artificial lamps dazzle the eye or produce glare for the light sensor. At too high of a light intensity, overloading of the eye or of the light sensor can result in irreparable damage or destruction.
For the human eye, this not only means that within the image of the powerful light source only a super-bright shine is still visible and not any structures, but also that the image is swamped out or blanketed by the shine in an entire surrounding area.
These effects are not desired in welding work, where the welding flame swamps out, distorts or diffuses details of the weld. They can also be harmful to pilots and soldiers, for example, who are blinded by the sun, stun grenades or muzzle flashes and, consequently, are no longer able to follow the events taking place. Video cameras are also dazzled or blinded and, in fact, often to a much greater extent than the human eye. A reason for this is that the dynamic range of video cameras is smaller than that of the human eye. And, when CCDs (charge-coupled devices) are used, one also encounters the disturbing effect that pixels, illuminated with too high of a light intensity, tend to “overflow”. In other words, i.e., adjacent pixels are able to register an intensity far greater than that actually incident on them, thereby resulting in considerable imperfections in the image.
The eye has a natural intensity-dependent attenuation device, namely the iris, which protects the eye at a high light intensity by narrowing the pupil. Cameras and other light sensors can also be equipped with an intensity-dependent attenuation device, e.g., with an adjustable diaphragm. Another attenuation possibility provides for connecting filters in series, e.g., a pair of polarization filters, which are rotated with respect to one another by a specific intensity-dependent angle.
A disadvantage of attenuation devices of this kind arises from their inertia. In such, an effective glare protection, for example, against rapidly lighting-up or flashing bright light sources, such as lightning bolts, is not provided.
A further drawback of attenuation devices of this kind is that they only attenuate the light integrally, i.e., the entire field-of-view is uniformly darkened by a specific factor, so that bright zones and darker zones of the field-of-view are darkened by the same factor. This can mean that, to prevent a bright zone of the field-of-view, such as the sun, from blinding the viewer or the light sensor, the field-of-view must be darkened or dimmed to the point where nothing more is recognizable in darker zones of the field-of-view. In photography, this effect is termed the “against-the-light effect”.
For that reason, special attenuation devices have been developed which do not reduce the integral light intensity, but rather darken the field-of-view only locally, where high-intensity light actually falls, while the remaining zones are not darkened or are only slightly darkened. If the field-of-view includes the sun, for example, and the sky surrounding it, then, using such a special attenuation device, only the sun itself is substantially darkened, not, however, the sky, so that there is no more “against-the-light effect”. Such special attenuation devices are implemented with the aid of optically addressable, spatially resolving light modulators (OASLM).
A light modulator (OASLM) of this kind includes a birefringent layer, whose indicatrix rotates out of a rest-position direction by a specific angle in response to the application of an external electric field to the layer. It is assumed here that the magnitude of the electric field strength is E. The electric field is generated by applying a voltage to a plate capacitor, between whose plates the light modulator (OASLM) is located.
The polarity of the electric field is given by the polarity of the voltage. For that reason, the magnitude of the electric field strength can be +E or −E. The direction of rotation of the indicatrix out of its rest-position direction is dependent on the polarity of the electric field (+E or −E) and reverses when the polarity of the field-generating voltage is reversed. For one polarity reversal, however, the magnitude of the rotation remains unchanged.
The magnitude of the rotation depends not only on the electric field strength, but, in particular, also on the intensity of the light passing through the birefringent layer: the magnitude of the angle of rotation increases with the light intensity, however, at a given field strength, a specific maximum angle of rotation, referred to in the following as maximum angle, not being exceeded in response to further increasing light intensity. The maximum angle is dependent on the field strength. Therefore, the direction of the indicatrix is only the same over the entire surface of the light modulator when the light intensity is also uniformly distributed over this surface. Otherwise, zones having a differently directed indicatrix form in the light modulator; in zones of very great light intensity, it rotates by approximately the maximum angle, while in zones of low light intensity, only by small amounts as compared to the rest-position direction.
German Application No. DE-OS 196 16 323 A1 refers to utilizing this effect to manufacture an attenuation device that darkens the field-of-view locally, only where high-intensity light actually falls. Here, one takes advantage of the fact that the polarization direction of linearly polarized light, which passes through a λ/2 plate, is inverted with respect to the indicatrix of the birefringent material. For that reason, the thickness of the birefringent layer of the light modulator is selected in such a way as to enable the light modulator to act as a λ/2 plate. The strength of electric field E is selected so as to enable a maximum angle of 45° to be attained.
A polarizer is positioned upstream from the light modulator in such a way that the polarization direction of the light transmitted by passing through the polarizer forms an angle of 45° with the rest-position direction. In addition, an analyzer is placed downstream from the light modulator. It is situated so as to be crossed with respect to the polarizer by 90°.
An optical system is used to image a field-of-view onto the light modulator that contains, for example, a very bright light source against a dark background. Therefore, a bright spot, namely the image of the bright light source, and a dark zone, namely the image of the background, are formed on the light modulator.
The electric field E is only able to rotate the indicatrix of the light modulator in the area of the bright spot by the maximum angle, i.e., by 45°, out of the rest-position direction. Therefore, in the area of the bright spot, depending on the polarity of the electric field (+E or −E), the indicatrix is either at an angle of 0° or at an angle of 90° to the direction of polarization of the light that is incident on the polarizer.
As mentioned above, when passing through a λ/2 plate, the polarization direction is inverted with respect to the direction of the indicatrix. Since, in the area of the bright spot, the angle between the polarization direction of the incident light and the indicatrix is 0° or 90°, the polarization direction, in response to the inversion, either passes into itself or is rotated by 180°, so that the only light leaving the bright spot is light whose polarization is rotated with respect to the input polarization either not at all (0°) or by 180°. In both cases, i.e., for every polarity of the electric field (+E or −E), the analyzer, which is crossed relatively thereto, performs a filtering-out function, so that the image of the bright light source is completely suppressed.
Another situation arises for the dark background. The electric field (+E or −E) is only able to rotate the indicatrix of the light modulator in the area of the dark zone by a small angle out of the rest-position direction. In the area of the dark zone, the indicatrix forms an angle with the input polarization that, in each instance, varies only slightly from 45° for both polarities of the electric field (+E or −E). Therefore, as a result of the inversion with respect to the direction of the indicatrix, the input polarization is rotated by approximately twice this angle, thus by approximately 90°. The analyzer allows this polarization direction to pass through, so that the image of the dark background can be viewed with almost undiminished intensity.
Since the birefringent material of the light modulator is a liquid crystal, e.g., a nematic or smectic liquid crystal, an electrolysis, thus an electrochemical decomposition of the material, begins in response to the application of an electric field. To prevent this, instead of a constant field (+E or −E), an alternating field is applied by continually reversing the polarity of the field strength using a specific operating frequency of between +E and −E.
To the extent possible, the field strength preferably has a square-wave characteristic, so that it alternates between constant values +E and −E, the transition times preferably being kept as short as possible. As mentioned above, both for the two polarities of the electric field, i.e., both for +E as well as for −E, the above system also has the effect of suppressing the bright light source, not, however, the dark background. In this connection, the indicatrix in the area of the bright spot rotates with the operating frequency, with respect to the rest-position direction, back and forth between the positive and the negative maximum angle, which is equivalent in terms of absolute value (here ±45°).
The rotation of the molecules or molecular parts of the liquid crystal, whose orientation is decisive for the direction of the indicatrix, is encumbered, however, with relatively substantial inertia for all liquid crystals that can be used to attain large maximum angles of 45°. This means that the indicatrix exhibits a relatively long response time to change of light. Typical values may be approximately 1/100 seconds. Light sources that light up suddenly, such as lightning bolts, can, therefore, not be suppressed quickly enough to prevent dazzling of the eye or glare for the sensor.
In response to every polarity reversal of the electric field, the light-attenuating effect is temporarily lost, since the polarity reversal causes the indicatrix to also pass through the rest-position direction in the area of the bright spot. For that reason, a long response time entails the further disadvantage that, following each polarity reversal of the electric field, a relatively long time passes until the light-attenuating effect is achieved again.
The German Patent Application No. DE-OS 196 16 323 A1 refers to an attenuation device that can do without a maximum angle of 45° because two light modulators are arranged in series. Nevertheless, in this configuration as well, relatively large maximum angles are necessary, so that there is also the drawback here that only liquid crystals having a relatively slow response time are able to be used.
However, there are a number of types of liquid crystals whose indicatrix has a very short response time to change of light. Using these liquid crystals, it is not possible, however, to achieve maximum angles large enough to be used in the systems proposed by the German Application No. DE-OS 196 16 323 A1.
A further drawback of the mentioned attenuation devices is that only one polarization direction of the incident light is utilized, while the other polarization direction is filtered out at the input polarizer.